Various kinds of disc-like optical discs that can be optically recorded and/or reproduced are now commercially available on the market as optical recording and reproducing mediums. As such optical discs, there are known a read-only optical disc in which embossed pits corresponding to data are formed on a disc substrate in advance, a magneto-optical disc for recording data through the use of magneto-optic effect and a phase change optical disc for recording data through the use of phase change of a recording film.
Of these optical discs, in the writable optical disc such as the magneto-optical disc and the phase change optical disc, it is customary that grooves extending along recording tracks are formed on a disc substrate. “Grooves” are what might be called guide grooves formed along the recording tracks to enable mainly tracking servo to be effected. An opening end between the grooves is referred to as a “land”.
In the optical disc having the grooves formed thereon, tracking servo is effected in response to a tracking error signal based upon a push-pull signal obtained from light that had been reflected and diffracted on the grooves. The push-pull signal is obtained from two photodetectors when the two photodetectors symmetrically located at the center of track, for example, detect light reflected and diffracted on the grooves to calculate a difference between the outputs from the two photodetectors.
In these optical discs, high recording density has been achieved so far by increasing reproduction resolution of an optical pick-up mounted on a reproducing apparatus. From an optics standpoint, the reproduction resolution of the optical pick-up has been improved by shortening a wavelength λ of laser light for use in reproducing data or by increasing a numerical aperture NA of an objective lens for use in focusing laser light on the optical disc.
In the respective formats of a so-called write once CD-R of CD (Compact Disc), a rewritable DVD-R of magneto-optical disc, a write once DVD-R of DVD (Digital Versatile Disc) and a rewritable DVD-RW of DVD (the above-mentioned disc names are all registered trademarks of optical discs), a groove recording format for recording data on the groove has been proposed so far. In the respective formats of magneto-optical discs conforming to the ISO system, a land recording format for recording data on the land has been proposed so far.
On the other hand, in the DVD-RAM (Random Access Memory) and so on, as a method for realizing a high-density optical disc, there has been proposed so far a land groove recording system in which track density can be increased twice as high as that of the prior art by recording data on both of the grooves and the lands.
High-density optical discs such as a DVR (Digital Video Recordable) that have been developed as next-generation optical discs in recent years and a so-called μ (micro) Disc which is a miniaturized MD whose disk diameter is reduced use the land groove recording system. FIG. 5 schematically shows an example of the proposed format of the land groove recording system in which a groove 2 wobbles, i.e. winds in synchronism with the adjacent groove 2. Reference numeral 9 denotes a land. This wobble signal generates a clock and is for use as a synchronizing signal and the like. Data information is recorded on the lands 9 and the grooves 2 and hence recording density twice as high as that of the prior art can be achieved.
However, when the optical disc such as the DVD-RAM uses the land groove recording system, if the focal points are not properly adjusted in the land recording and the groove recording upon recording and playback, respectively, then optimum optical recording and reproducing characteristics will not be obtained. There is presented a defect that an optical system will become complicated in structure.
As is reported in “ISOM 2000 Simulation Of Heat Generation And Conduction On Land/Groove Disc” in which recording beam shapes differ from each other in the land recording and in the groove recording, it is difficult to make the land recording and reproducing characteristic and the groove recording and reproducing characteristic become uniform. As a result, a problem arises in which the same optical recording and reproducing medium cannot avoid the regions which are different from each other in recording and reproducing characteristic.
Furthermore, we have experimental results in which while the high-density optical disc such as the DVR has a satisfactory recording and reproducing characteristic on the side near the read-out surface, i.e. in the case of the DVR, on the land near the side irradiated with light, it is difficult to hold a satisfactory recording and reproducing characteristic on the side distant from the read-out surface, i.e. in the case of the DVR, on the groove distant from the side irradiated with light.
While the optical disk such as a DVD-ROM (Read Only Memory) disk is able to directly record a signal by this land groove recording system format in the present situation, there is a demand for making recording and reproducing characteristics become satisfactory and uniform in the land recording and in the groove recording.
However, in the high-density optical disk such as the DVR, since the groove portions are distant from the read-out surface as described above, it is difficult to make the recording and reproducing characteristic on the groove portion become satisfactory.
On the other hand, in the manufacturing process of the optical recording and reproducing medium, there can be considered a manufacturing method for manufacturing an optical recording and reproducing medium by inverting concave and convex patterns formed on a disc substrate. Specifically, in the ordinary manufacturing process of the optical recording and reproducing medium, after very small concave and convex patterns had been formed on a photosensitive layer on a glass master by a suitable method such as photolithography, a master stamper made of Ni, for example, is formed by a suitable means such as plating.
Then, by an injection molding method for injection-molding this master stamper set on a mold and so on or by a so-called 2P (Photo-Polymerization) method for forming target concave and convex patterns by pressing the stamper against a resin layer obtained after a ultraviolet-curing resin had been coated on a disc substrate, it is possible to form an optical recording and reproducing medium substrate in which predetermined very small concave and convex patterns had been formed on the substrate.
Therefore, as described above, when the groove portions are formed on the side distant from the read-out surface so that satisfactory recording and reproducing characteristics cannot be held, a duplicate stamper of the above-mentioned master stamper, i.e. mother stamper is transferred and formed by a suitable method such as electroplating, whereby concave and convex patterns are inverted and groove patterns are formed on the disc substrate at its side near the read-out side to improve recording and reproducing characteristics.
However, when the optical disc uses the groove recording format or the land recording format, if high recording density similar to that obtained by the land groove recording format is achieved, then track density twice as high as track density obtained by the land groove recording format should be achieved, i.e. track pitch should be halved so that an amplitude amount of a tracking servo signal such as a push-pull signal is caused to decrease. As a result, stable tracking servo becomes difficult and reproduction of a wobble signal becomes also difficult.
In the land groove recording format, for example, a track pitch is 0.60 μm, i.e. a land width is 0.30 μm and a groove width is 0.30 μm, whereby an amplitude of a push-pull signal is approximately 90%.
However, when similar recording density is achieved by the groove recording format, if the track pitch is selected to be 0.32 μm, then the amplitude of the push-pulls signal is approximately 18%.
In the conventional optical disc, when the track pitch is selected to be approximately twice to ⅔ times the track pitch corresponding to a cut-off frequency of an optical pick-up of a reproducing apparatus, stable tracking servo can be realized and reproduction of a stable wobble signal can be realized, whereby an amplitude of a tracking servo signal can be obtained as sufficient level.
A cut-off frequency is a frequency at which an amplitude of a reproduced signal becomes approximately zero. When λ assumes a wavelength of laser light for use in reproducing data and NA assumes a numerical aperture of an objective lens for focusing laser light on the optical disk, the cut-off frequency is expressed as 2NA/λ.
In the case of the above-mentioned DVR, since the numerical aperture NA=0.85 and the reproduced light wavelength λ=406 nm, the cut-off frequency (2NA/λ) is expressed as 4187/mm and a corresponding track pitch is obtained as 0.239 μm.
If the track pitch of the DVR is selected to be 0.32 μm, then a resultant track pitch becomes approximately 4/3 (0.32/0.239 is nearly equal to 1.339) so that a sufficient tracking servo signal amplitude, i.e. sufficient push-pull signal amplitude cannot be obtained.
While the recent high-density optical disk uses a push-pull signal as a tracking error signal, in order to carry out tracking servo stably, a push-pull signal amplitude ratio should be large sufficiently. For example, a push-pull signal amplitude ratio should be selected to be approximately larger than 0.14 by a means such as adjustment of focus position for maximizing a push-pull signal amplitude. Furthermore, it is also requested that a wobble signal should be reproduced stably.